Field
Embodiments relate to glass and glass ceramic compositions and in particular, to glass ceramic compositions having a combination of lithium disilicate and -spodumene crystalline phases that are compatible with conventional rolling, float and blow molding processes.
Technical Background
Lithium disilicate glass-ceramics in the SiO2—Li2O—K2O—ZnO—P2O5—Al2O3—ZrO2 system have been developed and sold for use as dental crowns, bridges, and overlays. Their microstructures of interlocking tabular crystals provide high mechanical strength and fracture toughness and excellent chemical durability. Compositions in this area were invented at Corning, Inc. and patented by Beall et al. in U.S. Pat. No. 5,219,799 (“the '799 patent”). The composition of Example 3 from the '799 patent is given in Table 1 as composition A.
TABLE 1Wt %ASiO274.3Al2O33.6Li2O15.4K2O3.3P2O53.4
While lithium disilicate glass ceramics such as composition A provide excellent mechanical properties, their precursor glasses are very fluid and difficult to adapt to many forming processes other than casting. Such glasses can have liquidus viscosities of 500-750 poise. For comparison, typical floated, rolled, press, or blow molded soda lime glasses, as well as rolled beta-quartz-based glass ceramics (used for cooktops), have liquidus viscosities approaching 10,000 poise. It would be very desirable to increase the viscosity of the precursor glass while retaining the good mechanical properties of a glass ceramic with lithium disilicate as a major phase.
In addition, known glass-based materials often exhibit intrinsic brittleness or low resistance to crack propagation. For example, an inherently low fracture toughness (e.g., 0.5-1.0 MPa·M1/2 for oxide glass and glass ceramics) makes oxide glass sensitive to the presence of small defects and flaws. As a comparison point, commercially available single-crystal substrates exhibit a fracture toughness value in the range from about 2.4 to about 4.5 MPa·M1/2. Chemical strengthening by, for example, ion exchange processes can provide some resistance to crack penetration at the surface of a glass or glass ceramic by imposing a compressive stress layer in the glass or glass ceramic to a depth (e.g., 50-100 μm) from the surface; however, the crack penetration resistance may be limited and is no longer effective once a crack propagates through the compressive stress layer into the bulk of the glass or glass ceramic. Improvement of the mechanical properties of glass-based materials, in particular with respect to damage resistance and fracture toughness, is an ongoing focus. Accordingly, it would also be very desirable to provide glass ceramics with improved damage resistance and fracture toughness.